The formation of silver halide grains is composed of two main steps, a nuclear formation (nucleation) and a crystal growth. In T. H. James, The Theory of the Photographic Process, 4th edition, page 89, published by Macmillan Co., 1977, it is disclosed that "[a]lthough crystallization is often considered to consist of two major processes, nucleation and growth, two additional processes occur under some conditions of photographic emulsion precipitation, Ostwald ripening and recrystallization. Nucleation is the process in which there is a population explosion of the number of crystals when entirely new crystals are created. Growth is the addition of new layers to crystals that are already present. Ostwald ripening occurs predominantly at higher temperature, in the presence of solvents, and when there is a wide distribution of grain sizes. Recrystallization is the process in which the composition of crystals changes. That is, since in the formation of silver halide grains, nuclei are formed at the beginning and the subsequent crystal growth mainly occurs on the existing nuclei only, the number of the silver halide grains does not increase during the growth of the grains.
Silver halide grains are generally produced by reacting an aqueous silver salt solution and an aqueous halide solution in an aqueous colloid solution contained in a reaction vessel. In this case, there is known a single jet process of placing an aqueous solution of a protective colloid, such as gelatin, and an aqueous halide solution in a reaction vessel and adding thereto an aqueous silver salt solution along with stirring for a certain time. Also known is a double jet process of placing an aqueous gelatin solution in a reaction vessel and simultaneously adding an aqueous halide solution and an aqueous silver salt solution each for a certain time. Upon comparing both of the processes with each other, in the double jet process, silver halide grains having a narrower grain size variation are obtained and, further, the halide composition can be desirably changed with the growth of the grains.
Also, it is known that the nucleus formation of silver halide grains is greatly changed by the concentration of silver ions (or halogen ions) in the reaction solutions, the concentration of a silver halide solvent, the supersaturation, the temperature, etc. In particular, the heterogeneity of a silver ion concentration or a halogen ion concentration caused by an aqueous silver salt solution and an aqueous halide solution added to a reaction vessel causes the variation of supersaturation and solubility in the reaction vessel by each concentration, thereby the nucleus formation rate differs to cause a heterogeneity in the silver halide crystal nuclei formed.
In order to avoid the occurrence of the heterogeneity described above, it is necessary to quickly and uniformly mix the aqueous silver salt solution and the aqueous halide solution being supplied to the aqueous colloid solution for homogenizing the silver ion concentration or the halogen ion concentration in the reaction vessel.
In a conventional process of adding an aqueous halide solution and an aqueous silver salt solution to the surface of an aqueous colloid solution in a reaction vessel, the portions having a high halogen ion concentration and a high silver ion concentration occur near the addition locations of the aqueous solutions, which makes it difficult to produce homogeneous silver halide grains. For improving the local deviation of the concentrations, there are known the techniques disclosed in U.S. Pat. Nos. 3,415,650 and 3,692,283 and British Patent No. 1,323,464.
In these processes, a hollow rotary mixer (filled with an aqueous colloid solution and being, preferably partitioned into upper and lower chambers by a disk-form plate) having slits in the cylindrical walls thereof, is disposed in a reaction vessel filled with an aqueous colloid solution in such a manner that the rotary axis is placed in the direction of gravity. Further, an aqueous halide solution and an aqueous silver salt solution are supplied into the mixer, which is rotating at a high speed, through conduits from the upper and lower open ends and mixed quickly to react the solutions (i.e., when the mixer is partitioned into the upper and lower chambers by a partition disk, the aqueous halide solution and the aqueous silver salt solution supplied to the upper and lower chambers, respectively, are diluted with the aqueous colloid solution filled in both the chambers and then quickly mixed near the outlet slit of the mixer to cause the reaction). The silver halide grains thus formed are discharged into the aqueous colloid solution in the reaction vessel by the centrifugal force caused by the rotation of the mixer to form silver halide grains.
On the other hand, JP-B-55-10545 (the term "JP-B" as used herein means an "examined published Japanese patent application") discloses a technique of improving the local deviation of the concentrations to prevent the occurrence of the heterogeneous growth of silver halide grains. The process is a technique of separately supplying an aqueous halide solution and an aqueous silver salt solution into a mixer filled with an aqueous colloid solution from the lower open end, the mixer being placed in a reaction vessel filled with an aqueous colloid solution, abruptly stirring and mixing the reaction solutions with a lower stirring blade (turbine propeller) provided in the mixer to grow silver halide grains, and immediately discharging the silver halide grains thus grown into the aqueous colloid solution in the reaction vessel from an upper opening of the mixer by means of an upper mixing blade provided in the upper portion of the aforesaid mixer.
Also, JP-A-57-92523 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a production process of silver halide grains for similarly preventing the occurrence of local heterogeneity of the concentrations. That is, there is disclosed a process of separately supplying an aqueous halide solution and an aqueous silver salt solution into a mixer filled with an aqueous colloid solution from a lower open end, the mixer being disposed in a reaction vessel filled with an aqueous colloid solution. The process further includes diluting both the reaction solutions with the aqueous colloid solution, abruptly stirring and mixing the reaction solutions by a lower stirring blade member provided in the mixer, and immediately discharging the silver halide grains thus grown into the aqueous colloid solution in the reaction vessel from an upper opening of the mixer. As a result, both the reaction solutions, diluted with the aqueous colloid solution as described above, are passed through a gap formed between the inside wall of the aforesaid mixer and the end of a blade of the aforesaid stirring blade member, without passing through gaps between the individual blades of the stirring blade member, so as to abruptly mix the reaction solutions due to the shearing effect in the aforesaid gap and thus cause the reaction to thereby grow silver halide grains.
However, although in the aforesaid processes, the occurrence of the local heterogeneity of the concentrations of silver ions and halogen ions in the reaction vessel can be surely prevented to a considerable extent, the heterogeneity of the concentrations still exists in the mixer and, in particular, a considerably large variation of the concentrations exists near the nozzles for supplying the aqueous silver salt solution and the aqueous halide solution, and near the lower portion and the stirring portion of the stirring blade member. Furthermore, the silver halide grains supplied to the mixer together with the protective colloid are passed through the portions having such a heterogeneous distribution of the concentrations and, more importantly, are rapidly grown in these portions. In other words, in these processes, the variation of the concentrations exists in the mixer and since the grain growth rapidly occurs in the mixer, the purpose of performing a homogeneous nucleus formation and a homogeneous grain growth of silver halide grains in a state having no variation of the concentrations has not been attained.
Furthermore, various attempts have been made for solving the problem of the heterogeneous distribution of the silver ion concentration and the halogen ion concentration by more complete mixing wherein a reaction vessel and a mixer are separately disposed and an aqueous silver salt solution and an aqueous halide solution are supplied to the mixer and abruptly mixed therein to form silver halide grains.
For example, U.S. Pat. No. 4,171,224 and JP-B-48-21045 disclose a process and an apparatus for circulating an aqueous colloid solution (containing silver halide grains) in a reaction vessel at the bottom of the reaction vessel by means of a pump, disposing a mixer in the circulating route, supplying an aqueous silver salt solution and an aqueous halide solution to the mixer, and abruptly mixing both the aqueous solutions in the mixer to form silver halide grains.
Also, U.S. Pat. No. 3,897,935 discloses a process of circulating an aqueous protective colloid solution (containing silver halide grains) in a reaction vessel at the bottom of the reaction vessel by means of a pump and adding an aqueous halide solution and an aqueous silver salt solution into the circulation system.
Furthermore, JP-A-53-47397 discloses a process and an apparatus for circulating an aqueous colloid solution (containing silver halide emulsion) in a reaction vessel by means of a pump, including first adding an aqueous alkali metal halide solution into the circulation system, and after diffusing the solution until the mixture becomes uniform, adding an aqueous silver halide solution into the system followed by a mixing step to form silver halide grains.
However, in these processes, while the flow rate of the aqueous solution being circulated in the reaction vessel and the stirring efficiency of the mixer can be separately changed, and the grain formation can be performed under a condition of a more homogeneous distribution of the concentrations, eventually, the silver halide crystals sent from the reaction vessel together with the aqueous colloid solution cause an abrupt grain growth at the inlets of the aqueous silver salt solution and the aqueous halide solution. Accordingly, it is practically impossible to prevent the formation of the variation of the concentrations at the mixing portion or near the inlets as in the case described above, and thus, the purpose of homogeneously forming silver halide grains in a state having no variation of the concentrations has not yet been attained.